(1) Field of the Invention
This invention relates to means that have been devised in order to absorb energy which would otherwise be passed between two members which are caused for example by an earthquake to move in cyclic manner relative to each other. It is therefore an object of the present invention to provide such an absorber, and a method of using such an absorber which will enable structural members or portions within a structure to be connected but in such a way that energy is absorbed to a reasonable degree before being passed from one to the other. Such an absorber preferably also is such that it can be replaced after prolonged use if the need arises.
(2) Description of the Prior Art
Energy absorbers, otherwise known as shock absorbers or dampers, are well known. There are many designs which will markedly reduce the energy, whether impact or cyclic, transmitted by forces of a few tens of newtons. These are, however, applications in which forces several orders greater than this may cause damage, especially when the structure oscillates. As one example only, we may consider the effect of a major earthquake or a high wind on a structure such as a tall building or a bridge. Forces now involved are tens of thousands of newtons or more, and it is well known that they may cause damage to the structure.
It is standard practice so to design structures that the energy transmitted to the building by such a natural phenomenon as an earthquake or a high wind is absorbed within the building by additional material which costs a considerable amount of money and is redundant from static considerations. To reduce this extra expense, attempts have been made to incorporate in the building special panels, commonly reinforced concrete, which are designed to be energy absorbers. It is known that the special concrete panels are expensive and that they will be destroyed in one or a few severe earthquakes or storms and will have to be replaced at appreciable expense. Various attempts have been made to prevent the transmission to the building of earthquake forces. In one such attempt, the building is mounted on rubber. It is often called a rubber damper but this is to a large extent a misnomer. A damper is an absorber of energy and a rubber support for a building absorbs very little energy, but operates as a spring.
Over the past few years, time has been spent developing means of absorbing energy of motion. These energy absorbers use the principle of twisting, bending or extruding a material to deform the device and hence absorb energy. Details of such developments can be found in the following list of publications, not all of which were published before the priority date of this patent specification:
(1) Robinson W H and Greenbank L R, "Properties of an Extrusion Energy Absorber", Bulletin of the New Zealand National Society for Earthquake Engineering, Vol. 8 p.187, 1975. PA1 (2) Robinson W H and Greenbank L R, "An Extrusion Energy Absorber Suitable for the Protection of Structures during an Earthquake", Journal of Earthquake Engineering and Structural Dynamics, 1976. PA1 (3) Skinner R K, Kelly J M, and Heine A J, "Hysteretic Dampers for Earthquake-resistant Structures", Journal of Earthquake Engineering & Structural Dynamics, Vol. 3, pp 287-296, 1975. PA1 (4) Robinson W H, "Longitudinal Extrusion Energy Absorber" New Zealand Pat. No. 165897, December 1972, Australian Patent No. 463143, December 1972, U.S Pat. No. 3,833,093, December 1972, Mexican Pat. No. 132942, January 1973. PA1 (5) Robinson W H, "Torsional Extrusion Energy Absorber" New Zealand Patent No. 172665, November 1974, U.S. Pat. No. 3,953,013, November 1974, British Patent Application No. 48532, November 1974. Japanese Patent application No. 133479, November 1974. PA1 (6) Robinson W H, "Sleeve for Extrusion Energy Absorber", New Zealand Patent No. 175766, October 1975. PA1 (7) Skinner, R I, Bycroft G N and McVerry G H, "A practical system for isolating Nuclear Power Plants from Earthquake Attack" Nuclear Engineering and Design 36 (1976) P287-297. PA1 (a) as the temperature increases the yield stress decreases and, therefore, the energy absorbed decreases, PA1 (b) the higher the temperature the more rapidly the lead will recover and recrystallise.
If a solid is taken cyclically beyond its elastic limit but short of failure, it will absorb energy. When used as a support for a building, or when used by incorporation in the structure of the building the energy transmitted to the building or through it, in the two cases, is converted into heat instead of being applied in a destructive fashion to the building. Materials are known which are useful in this way when deformed in torsion or in bending. No such energy absorbers are heretofore known which are so designed that the distortion of the energy absorbing element is in simple shear.